Electronic devices and components have found numerous applications in measurement and monitoring of chemical or biological reactions such as detecting concentration, presence and activity of particular ions, enzymes, antibodies, antigens, hormones and gases. One such electronic device is referred to as an Ion Sensitive Field Effect Transistor, often denoted in the relevant literature as ISFET. This device facilitates the measurement, for example, of a hydrogen ion concentration (i.e. pH) in a solution.
FIG. 1 is a schematic cross-section of a conventional ISFET. This device is similar to a MOSFET and comprises a semiconductor substrate 141, a source 147 and a drain 145. The source 147 is spaced from the drain 145 and both are located near the surface of substrate 141. A channel region 152 is located between the source 147 and the drain 145 in the substrate 141. Inter alia in the channel region 152, the surface of the substrate 141 is covered by an insulator 101 and an ion sensitive thin layer or membrane 150 on top of the insulator 101. When the ion sensitive membrane 150 is exposed to an ionic solution, the surface potential is changed to vary the conductance of the channel region 152. By measuring the current between the source 147 and the drain 145, information such as the ion concentration of a chemical or biological reaction can be derived.
Another structure has been demonstrated which uses a MOSFET structure and connects a gate poly and an ion sensitive membrane by metal. When the surface potential of the membrane is changed due to the ion concentration or pH value of the solution, the gate voltage of the MOSFET is also changed. Information about chemical and biological reactions can then be extracted by monitoring the electric signal from the MOSFET.
A conventional ISFET needs a micro-well or recess for the chemical solutions. In this conventional ISFET, the recess needs to be well connected to the channel area of the ISFET. Further, a discrete signal readout circuit is generally employed for analyzing the signal readout from the conventional ISFET. The discrete signal readout circuit is arranged separately (i.e. not on the same chip) and connected to the conventional ISFET. Normally a printed circuit board (PCB) or wiring is used for connecting the discrete read out circuit with the conventional ISFET. The inventors have appreciated that no techniques exist which combine a conventional ISFET with signal on chip (SOC) technology using a standard CMOS process.